There are some other ways of reducing CPU usage but they all have gameplay drawbacks.
Every non-player race/empire that is generated will increase the amount of calculations that need to be done. Every system that is generated, by being discovered by either you or an NPR, has a chance of generating a new NPR. There are a couple of ways you can reduce this.
You can lower the Non-player Race Generation Chance from the default of 30%.
You can also somewhat slow the rate of new systems being generated. Disable Real Star Systems and increase the Local System Gen. Chance and decrease Local System Gen. Spread.
You can reduce how often some calculations are done by increasing the Construction Cycle Time. Though many things, like construction and research, will take a few days longer.